Why is this Mizmor according to the Aleph Beis, just Nun is omitted?
Rashi (from Brachos 4b): David saw a harsh fall [in a verse that begins] with Nun - "Naflah v'Lo Sosif Kum Besulas Yisrael" (Amos 5:2). He put a support for it - "Somech Hashem l'Chol ha'Nofelim."
Radak (1): It is according to the Aleph Beis, because it is totally praise of Hashem. I do not know why there is no verse for Nun 1 .
Radak cites Brachos 4b, which gives the reason! Perhaps he felt that the reason there is not sufficient. (PF)
Who are "ha'Nofelim"?
Malbim: They are those who fell from their level via the Ma'arachah and their bad Mazal. Hashem supports them so they not fall, and changes their Mazal to be good.
Lev Eliyahu (Shemos p.193): Also those who fall in Ruchniyos, Hashem supports them lest they fall more 1 , and raises them.
Kidushin 20b: If one sold himself to be a servant of idolatry, perhaps we do not redeem him! It says (Vayikra 25:48) "Ge'ulah Tihyeh Lo". (PF)
Who are "ha'Kefufim"?
Radak: They are the downtrodden poor. In His mercy, He supports and straightens them - "me'Ashpos Yarim Evyon; Lehoshivi Im Nedivim..." (113:7-8).
Malbim: They are the poor, who bend themselves. Hashem straightens them, i.e. He elevates them so they will not go bent.
